The present invention relates generally to the field of logic integrated circuits and, more particularly, to a new DC-DC converter with synchronous rectification that prevents or minimizes negative current build up.
The supply voltage of integrated circuits used in electronic devices, such as computer and communications equipment, has decreased from 5 volts to 2.5 volts or, in some cases, to 1.5 volts. Since the number of transistors in these integrated circuits has increased, the output current demand of the DC-DC converters has increased accordingly. The low output voltage DC-DC converter with the traditional Schottky diode rectifiers suffers from low efficiency and high power loss. Synchronous rectification technology has improved the efficiency of DC-DC converters significantly, especially as related to the low voltage output. However, the synchronous rectification has added more complexity to the DC-DC converter circuit. It has also made the system design more difficult because the DC-DC converter with rectification can sink current from secondary to primary. Because of this, the synchronous rectifiers have a common problem: they need a parallel tool converter, such as O-ring diodes, to allow parallel connection of the converters. Without the O-ring diode, one converter can sink the current from another converter, possibly destroying the second converter because the current can go as high as 20 or 30 amps. The O-ring diode restricts the direction of the current. However, the O-ring diode decreases system efficiency.
One technique to prevent the synchronous rectifier DC-DC converters from sinking current from the secondary involves monitoring the current through the synchronous rectifier. When this current reaches a predefined level, the converter turns off its synchronous rectifier and then relies on the body (switch) diode to conduct current. This technique works very well, but adds complexity to the circuit. Accordingly, there is a need for a simple circuit that will minimize or prevent the converter from sinking current.
The present invention provides a new circuit to enable to parallel the bridge type DC-DC converter with synchronous rectification. It is applicable to various types of DC-DC converters, including push-pull, full bridge and half bridge. The present invention provides a simple circuit that will minimize or prevent the converter from sinking current. As a result, the present invention eliminates unnecessary power dissipation, reduces potential damage to the converter and allows parallel operation of the converters. Although the present invention is described below in reference to a half bridge rectifier, the present invention can be implemented for full-bridge and push-pull topologies. In addition, the present invention is particularly useful in portable electronic devices, such as computers, personal data assistants and communication devices.
As an example, the operation of the present invention is described in relation to a half bridge DC-DC converter. When there is no load on the secondary, the average current through the output inductor is zero. In the free wheeling stage, both synchronous rectifiers are on, allowing the inductor current to flow in the negative direction. When there is a voltage source higher than the converter""s regulated voltage, the converter tries to reduce its duty cycle. Therefore, the primary MOSFET is turned off and the secondary MOSFET is turned on. At this point, the secondary rectifiers operate as if they are in the free wheeling stage. The duty cycle of the off state for the primary MOSFET is longer than the normal operation, allowing the inductor current to build up in the negative direction. The current in the transformer flows in the negative direction with respect to its normal operation. Once the primary MOSFET is turned off, the current flows through its body diode. At this time, the polarity and magnitude of the transformer is the same as before. The converter is sinking current from the secondary to the primary. In order to prevent the converter from sinking current from the secondary, the current must not be allowed to build up in the negative direction. This can be accomplished by adding a pair of switches.
The additional switches turn off the synchronous rectifiers at the appropriate time to prevent the negative current buildup in the inductor. When the current feeds back from the secondary to the primary, the effective duty cycle of the transformer is actually higher than the duty cycle of the converter feedback from the primary. By using that condition as a signal to connect to a switch that pulls the gate voltage of the synchronous rectifiers low, the current feedback from the secondary to the primary can be kept from reaching a significant level. There may still be some feedback. However, it should not exceed a half amp. This enables paralleling without the O-ring diode.
The present invention thus provides a method for minimizing or preventing a build up of a negative current in a DC-DC converter having a first secondary winding coupled to a first synchronous rectifier and a second secondary winding coupled to a second synchronous rectifier by turning the first synchronous rectifier off when the negative current is present in the first secondary winding and the first synchronous rectifier is on, and turning the second synchronous rectifier off when the negative current is present in the second secondary winding and the second synchronous rectifier is on.
The present invention also provides a DC-DC converter having a DC source, a first capacitor and a second capacitor series coupled across the DC source, a first primary switch and a second primary switch series coupled across the DC source. A primary winding of a transformer is coupled between the first and second capacitors, and the first and second primary switches. A first and second secondary winding of the transformer are coupled together. A first synchronous rectifier is coupled between the first secondary winding and a ground, and a second synchronous rectifier is coupled between the second secondary winding and the ground. An external driver is coupled to and provides timing signals to the first primary switch, the second primary switch and a DC level shifter. A first totem pole driver is coupled between the DC level shifter and the first synchronous rectifier, and a second totem pole driver is coupled between the DC level shifter and the second synchronous rectifier. A first circuit is coupled to the external driver, a first totem pole driver and the first secondary winding wherein the first circuit turns the first synchronous rectifier off when a negative current is present in the first secondary winding and the first synchronous rectifier is on. A second circuit is coupled to the external driver, a second totem pole driver and the second secondary winding wherein the second circuit turns the first synchronous rectifier off when the negative current is present in the second secondary winding and the second synchronous rectifier is on.
Other features and advantages of the present invention will be apparent to those of ordinary skill in the art upon reference to the following detailed description taken in conjunction with the accompanying drawings.